The present invention relates to a copying machine of the type in which the surface of an original is irradiated by a fluorescent lamp for exposure, an electrostatic latent image is formed on a light sensitive body, and the thus formed electrostatic latent image is transferred to a copy sheet.
Conventionally, the light output of fluorescent lamps used for exposure varies according to the tube wall temperature.
This is thought to be caused by the fact that the evaporation pressure of mercury sealed in the fluorescent lamp varies in accordance with the temperature change. The light output decreases when the wall temperature is either higher or lower than the optimum value.
Therefore, the tube wall temperature is required to be maintained at the optimum value. However, if the fluorescent lamp is turned on, the tube wall temperature inevitably rises, which makes keeping the tube wall temperature at the optimum value virtually impossible.
When the fluorescent lamp, having a tube wall temperature lower than the optimum value, is turned on, the temperature starts to go up and the light output gradually increases. Thereafter, the tube wall temperature exceeds the optimum value, i. e., an overheated state is attained, and the light output gradually decreases. This condition appears when a number of copies are reproduced from originals.
As the output of the light which is irradiated on the original changes, the amount of exposing light reaching the light sensitive body ultimately changes, which makes it difficult to produce copies of high quality.
One method suggested in the past for overcoming the above deficiency was to obtain predetermined amounts of exposing light by use of a slit or the like when the light output is the greatest value. However, in such a case, the background is stained on the first sheet of copy paper as well as on subsequent copies produced thereafter. Also, an image of low density on the original fails to be reproduced on copy paper at the greatest light output.
Another attempt at overcoming the shortcomings noted above was to heat the fluorescent lamp by means of a heating device or cooling by means of blowing air onto the fluorescent lamp, in order to maintain the tube wall temperatue at a predetermined level, thereby stabilizing the light output thereof. Although it is effective to blow air onto the fluorescent lamp to cool it off, a drawback appears in that relatively large equipment as well as space to install the equipment is required to accomplish the cooling effect.
It is accordingly an object of this invention to obtain optimum illumination during exposure from an illumination means.
Another object of the present invention is to conserve energy in copying machine use.
Another object is to maintain illumination only during exposure.
Another object is to obtain constant illumination from a lamp source during a reproduction process.
A still further object of the invention is to maintain the temperature of an illumination means at an optimum value.
These and other objects of the present invention are obtained by providing a copying machine in which the illumination source for exposure is turned off except when a necessary electrostatic latent image is being formed on the light sensitive body, i. e., when exposure is not required, the illumination means is turned off thereby preventing a rise in temperature and non-stabilization of the light output.